Perforating guns are well known in the oil and gas field. They are used to perforate a formation in a wellbore to stimulate production of hydrocarbons into the wellbore and ultimately to the surface. The typical process involves lowering a perforating gun on a tubing string and isolating the annulus between the tubing string and the wellbore with a packer. The gun may also be lowered into position by a wireline. The perforating gun is set through a variety of means. The explosive charge impinges on the wellbore and creates the perforations through which it is hoped the hydrocarbons will flow. The setting off of the perforating gun creates debris in the form of portions of the formation being displaced through the velocity of the explosive charge set off in the gun, as well as some of the explosive charge itself. It is desirable to get rid of the debris prior to beginning regular production from the wellbore. If the debris is not adequately removed, it can foul the recently created perforations and impede the removal of hydrocarbons from the formation to the surface through the wellbore. The presence of debris can also impede the insertion of screens and the performance of a commonly known procedure called "gravel packing."
In the past, various techniques have been used to remove such debris. One known technique is to create an underbalance adjacent the recently perforated formation. An underbalance is typically created by injection of a gas to displace some of the fluid in the wellbore to reduce the pressure adjacent the formation so that when flow is allowed to occur, the pressure adjacent the wellbore is reduced and formation fluids tend to flow into the wellbore rather than the reverse. Regardless whether circulation or reverse circulation is used, whether in conjunction with creating an underbalance or without, a potential problem of fluid losses into the formation exists. Additional problems exist if the velocity of the circulated fluid is insufficient to entrain some of the debris which is desired to be removed. In deviated wellbores the perforation may be over a long distance and existing equipment may be insufficient to create zones of sufficient velocity over the length of the perforation so as to be able to entrain the debris for its removal. Further problems can occur when, despite the fact that the circulation or reverse circulation is of sufficient velocity, there are pockets of low flow or no flow adjacent the recently perforated formation. This can result in erratic performance with regard to debris removal leaving some portions of the recently perforated formation cleared of debris while others are covered with debris. What has been lacking in the past is an ability to isolate small portions of the recently perforated formation and ensure sufficient velocities in those smaller portions to effectuate a more thorough removal of debris from those zones. What is also lacking from prior methods is a way to restrict or reduce, if not eliminate, the loss of fluids into the formation which can result from various operations typically performed after perforation such as bullheading.
To address these needs, the apparatus and method of the present invention have been developed so that small portions of the recently perforated zone can be isolated. Debris can be efficiently removed from these isolated zones through the use of a vacuum source which creates the requisite velocity to entrain the debris and bring it to the surface.
Jet pumps have been used in downhole applications in the past. Jet pumps achieve pumping action by means of momentum transfer between the power fluid and the produced fluid. In typical prior applications, the power fluid enters the top of the pump from the pump tubing and passes through a nozzle where virtually all of the total pressure of the power fluid is converted to a velocity head. The jet from the nozzle discharges into the production inlet chamber, which is in fluid communication with the wellbore. The production fluid is entrained by the power fluid and the combined fluids enter the throat of the pump.
The throat, which is always of a larger diameter than the nozzle, is where the mixing of the power fluid and production fluid takes place. The power fluid losses momentum and energy and the production fluid gains momentum and energy. The mixed fluid exiting the throat has sufficient total head to flow against the production return column gradient. Much of the total head is still in the form of a velocity head. The final working section of a jet pump is a shaped diffuser section of expanding area which converts the velocity head into a static pressure head greater than the static column head to allow flow to the surface.
Jet pumps have the advantage of not having closely fitting reciprocating parts, which allow them to tolerate power and production fluids of poorer quality than those normally required for reasonable life in a subsurface hydraulic pump. Jet pumps also have low profiles which make them adaptable for use in wellbores. Jet pumps can move higher volumes of liquid or gas as compared to conventional subsurface hydraulic pumps located in the same size tubing.
Jet pumps have been used in some high volume gassy or dirty wells. Such pumps are not applicable to all wells and are limited by their characteristics which require a relatively high suction pressure to avoid cavitation, and relatively low mechanical efficiency which requires higher input horsepower than a conventional hydraulic pump. Typically, such pumps have been used to enhance production from wells which need a downhole power assist for the hydrocarbons to be produced at the surface.